EP0430144B1 - Method and apparatus for reducing the concentration of nitrogen oxides in waste gases of combustion processes - Google Patents

Abstract

The method for reducing the nitric oxide concentration in the waste gas flow of combustion processes consists of the introduction of chemicals, e.g. ammonia, sal ammoniac, urea, calcium cyanamide, into the waste gas flow, wherein the waste gas flow is marked by an additional compulsorily produced strong turbulence and the waste gas flow is compressed prior to this turbulence zone. The introduction of the treatment medium is effected shortly before the turbulence zone and the waste gas flow is relaxed and calmed subsequent to the turbulence zone. In order to implement this method, a furnace is provided which comprises a grate and a waste gas flue consisting of an ascending waste gas flue and a descending waste gas flue adjoining the latter. The current flow of the waste gas flow is substantially S-shaped proceeding from the furnace grate to the downwardly directed waste gas flue, wherein the change of direction in the lower area is substantially smoother than in the reversing space. In the upper portion the waste gas flue the waste gas flow is compressed by means of corresponding inclination and consequent constriction of the cross section of the waste gas flue. Spray nozzles are provided at the commencement of the upper portion of the ascending waste gas flue in order to introduce chemicals. The introduction of the chemicals is effected shortly before the compression and the production of strong turbulence.

Description

The invention relates to a method and a device for reducing the nitrogen oxide concentration in the exhaust gas stream from combustion processes.

To reduce the nitrogen oxide concentration in the exhaust gas stream from combustion processes, it is known to use various chemicals, e.g. Introduce ammonia, ammonia, urea, calcium cyanamide into the exhaust gas stream of an incineration plant. So far, the main focus of all these processes has been on the supply of chemicals in certain temperature ranges. In order to achieve useful results, the chemicals were introduced at several points, the composition and concentration of the chemical solutions or chemical suspensions being matched to the temperature conditions prevailing in the different introduction levels. In order to achieve a good distribution of the chemicals introduced, a large number of insertion points or injection nozzles have been provided. These measures are not only structurally complex, but also require a great deal of control engineering.

The problem caused by the large number of introduction points is alleviated to a certain extent by the combustion system known from DE-C-37 22 523 with nozzles for blowing in ammonia for selective, non-catalytic smoke gas denitrification. In this known combustion system with a vertical flue gas flue, the flue gases rise evenly upwards without being influenced. The lower ends of the front and rear boundary wall of the exhaust duct are inclined in the sense of a cross-sectional constriction in order to collect the flue gases and into the vertical Direction. Height-adjustable nozzles for spraying treatment medium are provided within the evenly rising exhaust gas flow, the adjustability of the nozzles eliminating the need for a larger number of fixed nozzle levels, thereby avoiding the many supply lines to the individual nozzle levels. The adjustability of the nozzles is provided in order to be able to adapt the inlet level to the prevailing temperature conditions in accordance with the load on the combustion system. However, in order to bring about a sufficient reaction between the ammonia and the flue gases in this steadily rising flue gas stream, a nozzle holder with several distributor pipes and many blow-out nozzles is provided in order to achieve a widespread distribution of the treatment medium in the flue gas, which is also adjustable in height, what still requires a high level of construction, measurement technology and control technology to adapt the position of the nozzle holder to the prevailing temperature conditions.

In a method known from DE-A 35 10 669 for the adsorptive removal of sulfur oxides and other harmful substances from flue gases, in a device arranged outside the firing system, a rotational flow is initially caused in a circular cross-section entry chamber by tangential entry of the flue gases, which in the Has some turbulence inside. This chamber is followed by a venturi device, in the confuser of which the flue gases are compressed and accelerated vertically upwards. At the point of highest acceleration and compression, which is formed by the throat of the Venturi device, the treatment medium is introduced, whereupon the mixture is expanded in the subsequent diffuser and then passed into an ascending pipe shaft, in which the actual reaction occurs. Due to the compression and acceleration upward, the spiral flow is pulled apart, which leads to a uniform flow, so that the certain turbulence generation is no longer effective before the introduction of the medium to the desired extent. The compression zone is very short, so that there is no reaction here that only takes place in the smooth and calm flow. This well-known treatment method takes place in an independent of the furnace Device instead, so that an additional design effort is required especially for the flue gas treatment.

From US-A-4 722 287 a pulverized coal firing with an adjoining vertically rising exhaust gas flue is known, which in its upper region, in which the exhaust gas flow is deflected into the downward flue gas flue, has a nose which is essentially frustoconical in longitudinal section and which extends upwards into the flue gas duct, the axis of symmetry being approximately at an angle of 45 °, so that the rear wall of the nose delimiting the descending flue gas line is vertical. This formation of a nose causes a congestion effect and associated compression of the exhaust gas flow, but the overall design of this nose is designed for a largely turbulence-free deflection of the exhaust gas flow. A treatment medium for reducing the sulfur oxide content is introduced in the region of the uniformly rising exhaust gas stream before the later compression by the nose. For the introduction of a treatment medium, nozzles are provided on opposite walls of the exhaust gas duct, the nozzles having an inner tube with an outer deflecting body around which an annular channel is formed. The treatment medium is blown into the exhaust gas flow both through the ring channel and through the inner tube. Due to the deflecting body seated on the inner tube, the outer annular jet is guided as close as possible to the walls of the exhaust gas flue, while the inner tube blows the treatment medium far into the interior of the exhaust gas flue, which is to achieve thorough mixing across the entire cross section of the exhaust gas flue. This not only requires a relatively large number of nozzles in order to achieve thorough mixing, but the adjustment of the nozzles, which is effected by displacing the inner tube provided with the deflecting body with respect to the outer tube, requires a correspondingly high level of control engineering effort.

The object of the invention is, on the one hand, to reduce the structural and control-related outlay and, on the other hand, to further reduce the nitrogen oxide concentration.

This object is achieved by the method steps specified in claim 1.

The fact that the treatment medium is introduced at the beginning of the strong compression of the exhaust gas stream results in a higher density of the molecules to be influenced by the chemicals in relation to the exhaust gas volume, which increases the likelihood of the introduced chemicals having an effect on the molecules to be influenced. In this area of steadily increasing compression, a strong lateral acceleration occurs due to the narrowing of the cross-section and the associated arcuate guidance of the exhaust gas flow, so that an intensive mixing of the two media can take place as a result. The increasing compression with increasing transverse acceleration thus leads to a significantly improved reaction between the treatment medium and the exhaust gas flow in this upper region of the upward exhaust gas train, this reaction then being increased by the subsequent relaxation with a significant increase in turbulence. The creation of particularly good reaction conditions, which is further increased by the increasing compression and lateral acceleration combined with good mixing and the subsequent expansion and increase in turbulence, is achieved in the present invention not by a large number of nozzles, but by fluidic measures which are simpler due to simpler structural designs of the exhaust flue can be achieved. This makes it possible to reduce the number of insertion points or insertion nozzles and the associated outlay at the outset.

An improvement in the homogenization of the exhaust gas flow can be achieved in a further development of the invention in that the exhaust gas flow is introduced obliquely to the vertical by inclination of the rear boundary wall of the exhaust gas train in the sense of a cross-sectional widening and then in a further section with slight compression by reducing the cross-sectional area in when the exhaust gas flue enters the plumb line is aligned before it enters the heavy compression section. Due to the exhaust gas routing according to the invention, which is essentially S-shaped down into the downward exhaust gas duct Has course, the first arcuate course up to the deflection edge of the ascending exhaust duct in a much gentler arc than the adjoining deflection curve, is initially a certain calming through the cross-sectional expansion in the lowest area of the exhaust gas flue and through the oblique introduction of the exhaust gas flow into the exhaust duct and equalization of the exhaust gas.

The generation of the highest turbulence is advantageously achieved by deflecting the exhaust gas flow around the upper end of the rear boundary wall. The construction effort is particularly low, since it is only necessary to incline the upper part of the rear boundary wall in the sense of a cross-sectional constriction and to let it run out in a smooth edge, which results in a sharp deflection curve which leads to the desired turbulence. Due to the formation of a tear-off edge of the flow at the upper end of the rear boundary wall, the nose usually provided in known combustion plants for suppressing turbulence is eliminated. Due to the inclined position of the upper part of the rear boundary wall, the cross section of the downward flue gas flue is enlarged, so that the space required for the desired relaxation is created without additional structural measures. Since the changes in direction of the exhaust gas flow can be achieved in a simple manner by appropriately designing the combustion chamber or exhaust gas flue, the effort required to achieve particularly strong turbulence and the subsequent expansion is extremely low.

However, it is also possible to generate the turbulence by introducing an additional gas into the exhaust gas flow.

Furthermore, it is possible with simple means to generate the turbulence by internals within the flow path of the exhaust gas stream.

If, in a further embodiment of the invention, the treatment medium is introduced in aqueous solution by means of a single-substance nozzle, the energy expenditure for the introduction of the treatment medium is compared to the introduction of the treatment medium by means of two-substance nozzles lowered, since no additional carrier and atomizing medium is required, which requires a corresponding amount of energy for its provision.

In order to increase the possibility of the treatment medium having an effect on the molecules to be reduced, it is advantageous if the treatment medium is introduced into the exhaust gas stream in droplets of such size that an explosive evaporation of the solvent or carrier medium is possible.

According to the invention, a device, that is to say a firing system for carrying out the method, having an upward-directed exhaust gas flue arranged above a firing grate with fuel feed and slag discharge, a reversing space for the exhaust gas flow and a downward-directed exhaust gas flue are characterized in that the ascending flue gas line is divided into three sections is divided, in which the boundary wall facing the slag discharge is inclined in the first section in the sense of a cross-sectional widening, in the second section essentially vertical, and in the third section starting from the vertical in the direction of the fuel feed in the sense of a cross-sectional constriction that the cross-section is inclined of the first Section is slightly expanded towards the second section and the cross section of the third section is narrowed compared to the cross section of the second section, and that spray nozzles for introducing a treatment medium are arranged in the lower region of the third section.

With this simple constructional measure, an essentially S-shaped flow of the exhaust gas flow is achieved, starting from the firing grate up to the downward flue gas flue, the change in direction in the lower area of the flue gas flue being far less pronounced than is the case in the reversing chamber. In connection with the strong narrowing of the third section of the exhaust gas flue, the reversal of direction creates a strong turbulence and a thorough mixing of the exhaust gas flow and thus the advantageous conditions for the effects of the chemicals on the nitrogen monoxide.

If the front wall, i.e. H. the wall of the flue gas duct facing the firing object is designed to be vertical in order to simplify the construction, it is advisable in a further development of the invention to provide at least one injection opening for secondary air at the beginning of the first section of the flue gas on the boundary wall facing the firing object, whereby the essentially S- Flow course of the exhaust gas stream is supported.

The invention is explained in more detail below with reference to an exemplary embodiment of a firing system shown in the drawing, which is shown in the accompanying single figure.

As can be seen from the drawing, which shows a section through a firing system in a highly schematic form, it has a firing material feed, generally designated 1, which comprises a feed hopper 2, a feed table 3 and feed piston 4 which can be moved back and forth. By the loading piston 4, the kiln, z. B. Garbage placed on a grate 5, which is supplied with combustion air from below, which is fed to the grate by means of a blower 6 via a feed line 7.

The combustion of the fired material designated 8 takes place on the grate 5, the combustion zone, which is also the production zone of the nitrogen monoxide to be reduced, extending somewhat over half the grate length. The remaining part of the grate serves to cool the slag, which falls into a slag discharge 9 at the end of the grate.

The firing grate 5, starting from the firing material feed 1, is inclined in the direction of the slag discharge 9 and is preferably designed as a push-back grate.

Above the combustion zone of the firing grate 5, an exhaust gas flue, designated overall by 10, extends upwards, which opens into a reversing space 11, to which an exhaust gas flue 12 directed downwards connects. This exhaust flue is followed by an upward flue 13, the coarsest dust particles being discharged down onto the grate due to the strong deflection.

The first rising flue gas duct 10, which receives the flames arising during combustion in its lower region, also serves as a combustion chamber and is essentially divided into three sections 14, 15 and 16, which are delimited by lines 17, 18 and 19. The boundary wall of the exhaust gas flue facing the charge 1 is designated by 20 and the rear boundary wall facing the slag discharge 9 is designated by 21.

The rear boundary wall of the flue gas duct, which is designated overall by 21, is inclined in the first section 14 of the flue gas duct, in which it is designated by 22, starting from the vertical in the direction of the slag discharge 9, so that the cross section of the first is widened Section 14 results in the direction of the second section 15. The second section 15 is essentially delimited by parallel walls, while the rear wall of the third section 16, which is designated by 23, is inclined relatively strongly from the vertical in the direction of the firing object 1, so that there is an increasing sharp narrowing of the cross section of the third section 16 results. The cross section of the third section 16 is substantially narrowed compared to the cross section of the section 15. This results in a strong compression of the exhaust gas flow. The end of the rear boundary wall 23 projects into the reversing space 11. The exhaust gas flow is directed in a sharp arc around the edge 24 of the boundary wall 23 and enters the downward exhaust duct 12. Since the rear boundary wall 23 is at the same time the boundary wall for the downward exhaust duct 12, this is wider in its upper part than in its lower part, the upper region of the exhaust duct 12 in Cross-section is substantially larger than the upper region of the third section 16 of the exhaust duct directed upwards. This results in a relaxation and calming of the exhaust gas flow after flowing around the edge 24.

Due to the described design, there is an essentially S-shaped flow profile of the exhaust gas flow, starting from the combustion grate up to the exhaust gas duct 12 directed downwards, the flow profile being indicated schematically by the dash-dotted line 25. Due to this S-shaped course, the exhaust gas flow is subjected to two changes of direction, the first change of direction taking place at the transition from the first section 14 to the second section 15, while the second change of direction is already initiated in the third section 16 and then takes place in a sharp arc in the reversing space 11 continues. The flow pattern is therefore subject to a smaller change in direction in the lower region than in the upper region.

In the lower area of the third section 16, in which the compression of the exhaust gas stream begins due to the inclined boundary wall 23, spray nozzles 26 are arranged, through which chemicals in aqueous solution or suspension are sprayed into the exhaust gas train in a finely divided form, the spray nozzles preferably are designed as single-substance nozzles. The atomization of the aqueous chemical solution or suspension caused by the nozzles leads to an explosive evaporation of the water, so that the chemicals introduced are present in solid form in the flue gas immediately after the injection. As already mentioned at the beginning, ammonia, ammonia, Urea or calcium cyanamide in aqueous solution or suspension are sprayed in to bring about the reduction of the nitrogen monoxide. Due to the change of direction introduced in the third section 16 in connection with the strong compression of the exhaust gas flow and the very strong change of direction around the edge 24, a particularly strong turbulence is impressed on the exhaust gas flow, in which the conditions for the reduction of the nitrogen monoxide are particularly good, whereby due to the compression in section 16, the density of the molecules to be influenced relative to the gas volume is greater than in the rest of the exhaust gas stream, which likewise leads to an improvement in the conditions for the reduction of the nitrogen monoxide. Due to the arcuate course of the flow path 25 in section 15, the gas molecules are accelerated in the direction of the boundary wall 23, which in connection with the reversal of direction introduced by this boundary wall 23 leads to an acceleration of the gas particles in the opposite direction, as a result of which the gas molecules of the gas molecules are mixed thoroughly imported chemicals is brought about. The compression of the gas flow and the generation of a particularly strong turbulence then lead to an even greater reduction in the nitrogen oxide content in the exhaust gas flow than was possible with the previously customary methods.

To the exhaust gas flow immediately above the grate 5 in the desired direction. d. H. In the direction of the boundary wall 21, a blow-out nozzle 27 for secondary air is provided at the lower end of the front boundary wall 20. As a result, the boundary wall 20 can be designed as a simple vertical boundary wall.

Claims (9)

1. Method for reducing the concentration of nitrogen oxides in waste gases of combustion processes, in which a treating agent is introduced into the ascending waste gas stream within a waste gas offtake of the combustion plant, in which the waste gas stream is deflected by inclining the uppermost section of the rear boundary wall, in that the cross section is narrowed sharply, and is thus subjected to thorough mixing by transverse acceleration with increasingly high compression, the treating agent being introduced in the region of the beginning of high compression, after which the highest turbulence is produced in the waste gas stream already loaded with treating agent at the end of compression, with expansion beginning simultaneously.
2. Method according to claim 1, characterised in that the waste gas stream is introduced at an incline with respect to the vertical when it enters the waste gas offtake by inclining the rear boundary wall of the waste gas offtake, in that the cross section is widened, and is then aligned with the vertical in a further section with slight compression by a reduction of the cross section before it enters the section of high compression.
3. Method according to claim 1 or claim 2, characterised in that the highest turbulence is produced by deflecting the waste gas stream about the upper end of the rear boundary wall.
4. Method according to one of claims 1 to 3, characterised in that the turbulence is produced by introducing an additional gas into the waste gas stream.
5. Method according to one of claims 1 to 4, characterised in that the turbulence is produced by means of baffles within the flow path of the waste gas stream.
6. Method according to one of claims 1 to 5, characterised in that the treating agent is introduced in aqueous solution by means of a single-component nozzle.
7. Method according to one of claims 1 to 6, characterised in that the treating agent is introduced into the waste gas stream in drops of such a size as to allow for explosion-like evaporation of the solvent or carrier medium.
8. Apparatus for reducing the concentration of nitrogen oxides in waste gases of combustion processes, comprising an upwardly directed waste gas offtake (10) arranged above a grate (5) with an inlet (1) for the material to be burnt and a slag outlet (9), a reversing space (11) for the waste gas stream and a downwardly directed waste gas offtake (12), characterised in that the ascending waste gas offtake (10) is divided into three sections (14, 15, 16), in which the boundary wall (21) directed towards the slag outlet (9) is inclined in the first section (14), in that the cross section is widened, is vertical in the second section (15) and, in the third section (16), departing from the vertical, is inclined in the direction of the inlet (1) for the material to be burnt, in that the cross section is narrowed, that the cross section of the first section (14) is slightly widened in the direction of the second section (15) and the cross section of the third section (16) is narrowed sharply with respect to the cross section of the second section (15), and that spray nozzles for the introduction of a treating agent are arranged in the lower region of the third section (16).
9. Apparatus according to claim 8, characterised in that at least one injection opening (27) for secondary air is provided at the beginning of the first section (14) of the waste gas offtake (10) at the boundary wall (20) directed towards the inlet (1) for the material to be burnt.

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